In semiconductor fabrication, various layers of insulating material, semiconducting material and conducting material are formed to produce a multilayer semiconductor device. The layers are patterned to create features that taken together, form elements such as transistors, capacitors, and resistors. These elements are then interconnected to achieve a desired electrical function, thereby producing an integrated circuit (IC) device. The formation and patterning of the various device layers may be accomplished using various fabrication techniques including oxidation, implantation, deposition, epitaxial growth of silicon, lithography, etching, and planarization.
One important etching procedure for forming semiconductor features includes reactive ion etching (RIE) performed by a plasma of reactive ions. In the field of plasma etching, various processing parameters including, for example, RF power, RF bias, pressure, gas flow rate, temperature, and vacuum conditions are pre-determine in order to accomplish a particular plasma etching process. In addition, many plasma etching processes are pre-programmed to follow a series of steps for predetermined time periods also referred to as a process recipe, where the processing parameters may be altered at each step to achieve a desired etching result.
In addition, the plasma processing system frequently includes an endpoint detection means to determine an etching endpoint. For example, in dry etching the etching process frequently does not have good selectivity to an underlying material layer making an endpoint detection system necessary to avoid overetching of an underlying layer. Endpoint detection systems monitor parameters such as a change in the etch rate, the type of etch products, or a change in the active reactants in the gas discharge. For example, and optical emission spectroscopy (OES) has been widely used for endpoint detection by monitoring the intensity of wavelengths of either reactive species or etch products. More specifically, during an RIE process, plasma discharge materials, such as etchant, neutral, and reactive ions in the plasma, are continuously excited by electrons and collisions, giving off emissions ranging from ultraviolet to infrared radiation. An optical emission spectrometer diffracts this light into its component wavelengths and determines the intensity at a particular wavelength. Since each species emits light at a wavelength characteristic only of that species, it is possible to associate a certain wavelength with a particular species, and to use this information to detect an etching endpoint.
Also useful for monitoring reactants and products in a plasma etching process includes mass spectroscopy, where a molecule is broken into fragments and then analyzed typically using a magnetic field to determine the mass of the constituent fragments.
In forming a feature in a layer of material in a multilayer semiconductor device, an etching process typically etches away one type of material included in one layer of the device until another different layer of material, which typically has a low selectivity to etching, is reached, for example, an etch stop layer. Since the etch stop layer is typically a different material having a low reactivity with the reactive ion species, a discernible change in either or both the concentration of reactive ion species and the reactively etched species may be easily observed. Since the concentration of given plasma species is proportional to an emitted wavelength of light, the concentration of a given species may be tracked by monitoring the intensity at a given wavelength.
One problem with end-point detection using OES or any other plasma species monitoring method occurs where no change in material such as a material interface is present during the etching process thereby providing no readily discernable change in reactive species or reactively etched (product) species. There are several applications, however, where no material interface is present, yet a particular feature etching depth is required, for example, a trench or via etching depth a silicon or dielectric material.
Several approaches have been proposed to overcome the lack of a clearly discernible material interface including tracking the amount of either or both reactants and plasma etching product species present as a function of etching time during the etching process. For example, statistical techniques typically single out one or more individual plasma species components to monitor over time where the selection of the components is determined by statistical analysis of previous etching procedures. Alternatively, particular etching processes without a clearly discernible material interface rely on pre-determine time period for etching under particular plasma processing parameters.
One problem with the prior art plasma processing systems is the failure to provide a mechanism for plasma processing recovery in the event of an unexpected shutdown of the equipment, for example in the event of a power failure, hardware, or software failure. For example, in prior art plasma processing systems it is frequently difficult if not impractical to resume a plasma process, for example plasma etching where a clearly discernable interface is absent. For example, to resume a plasma process it is necessary to know the precise history of previous plasma processing conditions associated with a particular plasma etch recipe including time at a particular RF power as well as other processing and plasma species monitoring parameters. Prior art plasma processing systems have not provided a readily accessible and operable means to resume a plasma process, for example, a plasma etch process, following an unexpected shutdown of the process. As a result, process wafers must be scrapped in the event of an unexpected shutdown due to the impracticality of resuming a plasma process, particularly an etch process where a clearly discernable etching interface is absent.
These and other shortcomings demonstrate a need in the semiconductor processing art to develop a method for tracking a plasma process including a plasma etch process such that the plasma process may be easily resumed following a process shutdown.
It is therefore an object of the invention to provide a method for tracking a plasma process including a plasma etch process such that the plasma process may be easily resumed following a process shutdown while overcoming other shortcomings and deficiencies in the prior art.